In recent years, an increase in integration density in a semiconductor integrated circuit has resulted in an ever-increasing demand for an increase in fineness also in a reticle used in the preparation of this circuit. For example, for DRAM, the line width of a device pattern transferred from a reticle for 16 M DRAM is as small as 0.5 .mu.m. Further, a device pattern of 64 M DRAM requires a resolution of a line width of 0.35 .mu.m. Conventional light exposure systems using a stepper have reached their limit in terms of the ability to provide a further increased fineness.
This inability has led to studies on various methods that can increase the resolution of the device pattern to a level usable for practical use. Among others, a phase shift photomask that uses a conventional stepper exposure system and can increase the resolution of the device pattern transferred from the reticle has attracted attention.
Various phase shift photomasks have been proposed in the art and put to practical use, such as the photomasks shown in each of FIGS. 1a-1c. For the photomask of FIG. 1a, a transparent substrate 102 is provided with depressions 141 in every other space section among space sections between adjacent absorber layer portions 104 to shift a phase of the incoming light, for the photomask shown in FIG. 1a, transparent substrate 102 is engraved to change the phase of exposing light to half-wavelength, and this type of photomask is known as “substrate engraving type.” In this case, when a quartz substrate is used as the transparent substrate, the photomask is known as “quartz engraving type.” For the photomask of FIG. 1b and 1c, a SiO2 shifter layer 131 is provided onto the transparent substrate 102 to shift a phase of the incoming light. In the case of FIG. 1b, the SiO2 shifter layer 131 is provided between the transparent substrate 102 and absorber layer 104 in every other space section among space sections between adjacent absorber layer portions 104. In the case of FIG. 1c, absorber layer 104 has a repeated pattern and is provided on transparent substrate 102, and a shifter layer 131, which functions to change the phase of the exposing light to half-wavelength, is provided in every other space section among space sections between adjacent absorber layer portions 104. FIG. 1b shows a “Cr on shifter” type photomask wherein a SiO2 shifter layer 131 is provided under the light-shielding layer or absorber layer 104 to change the phase of the exposing light by a half-wavelength. FIG. 1c shows “shifter on Cr” type photomask wherein a SiO2 shifter layer 131 is provided on the top of the light-shielding layer or absorber layer 104 to change the phase of the exposing light by half-wavelength.
However, disadvantageously, prior art photomasks are not able to provide the imaging contrast needed to allow the reliable production microelectronic devices exhibiting densities required for next generation chips.
For simplicity and clarity of illustration, elements in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Where considered appropriate, reference numerals have been repeated among the drawings to indicate corresponding or analogous elements.